1) Start by making a circle. Hold down Shift while creating the circle if you want the arc to be non-elliptical.
2) Click Ungroup to remove any auto-grouping.
3) Click Decompose to convert the circle from a polycurve into four distinct curves. Each will be an arc that extends 90°; where the arcs touch, there will be a hollow black handle.
4) If you want an arc of 180° or 270°, then use Glue tool to glue some of the arcs back together. Move the glue bottle over the joints (at the North, South, East, and West points of the circle), click and release to glue the two adjacent arcs together.
5) Select and press Delete to delete the remaining portions of the circle that you no longer want.